Owing to the popularity of 12V halogen lamps, there is need for effective power supplies to convert mains voltage of 120V or 230V to 12V. One popular arrangement is the so-called "electronic transformer" in which the 50 Hz or 60 Hz utility current is rectified (giving 100 Hz or 120 Hz, respectively) and then inverted (i.e. chopped) using a half or full bridge topology to give a square wave with fundamental frequency typically in the range of at least 20 kHz.
Such systems have in the past suffered from a number of disadvantages. In the first instance, the necessity to protect the device against short circuits on the output, where implemented, involves passing the current in the system through a resistor such that when the current in the system exceeds a certain preset value the voltage across the resistor is of a value to switch on a transistor which in some way disables the system. However, in a 50 W system, for example, typically as much as 1 W is dissipated through the resistor causing unnecessary heating of the system. Such heating causes further inefficiencies as many components are less efficient when warm.
A second problem is that efficient use of the devices depends on an environment which conducts heat away from the electronic transformer in an effective manner. However, users of the transformer may be unaware of this requirement and may place the transformer within an enclosure containing trapped air or insulating materials or in a place where it may occasionally encounter high ambient temperatures. The known solution of using a temperature-sensing device to disable the electronic transformer at a certain preset temperature is often unacceptable as the transformer is required to work even when ambient temperatures are for some reason high.
A third problem relates to the use of electronic transformers where a lead-edge dimmer (e.g. an SCR, also called an incandescent dimmer owing to its popularity for use with line voltage incandescent lamps) is connected to the input. The dimmer reduces the power in the system by disconnecting the input for some proportion of the 50 Hz or 60 Hz half-cycle following the zero crossing (see FIG. 4c for a schematic of the input voltage to the electronic transformer when such a dimmer is in use). Each time the dimmer triggers, the input voltage jumps from zero to a substantial voltage and it is necessary for the inverter to be triggered in to oscillations very quickly in order to avoid the SCR device switching off (as will occur if there is no hold-up current for more than a few hundred microseconds). However, as the input voltage prior to dimmer triggering is zero, the capacitor providing energy for the inverter's triggering device is uncharged when the dimmer triggers and must take some time to charge before the inverter can trigger, during which time the SCR may switch off. This means that many electronic transformers cannot work when lead-edge dimmers are connected to the input.
A fourth problem relates to the use of electronic transformers, particularly of high power ratings (e.g. 300 W) where a dimmer is connected on the input even when the above problem is overcome, owing to the need also to design the transformer with radio frequency interference filters. This necessitates the use of a large capacitor after the input rectification of the system (before the inverter) where the frequency is 100 Hz or 120 Hz. However, when a dimmer is used, the sudden surge of current each time the dimmer triggers causes unacceptable acoustic sound in the capacitor (which, at 100 or 120 Hz is audible) and which may also damage the capacitor.
A fifth problem also relates to the use of electronic transformers of high power ratings. The inverter in an electronic transformer will generally stop oscillating when the input current crosses zero volts. Oscillations will only resume hundreds of microseconds later when the input voltage reaches a sufficient triggering threshold. As a result, the envelope of the output from an electronic transformer generally looks like FIG. 4 (a) having a break in each cycle of the rectified 50 Hz or 60 Hz envelope. This is associated with a harmonic distortion of the order of 20% which is unacceptable.